Communication devices such as mobile stations are also known as e.g. mobile terminals, wireless terminals and/or User Equipments (UEs). Mobile stations are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system. The communication may be performed e.g. between two mobile stations, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Mobile stations may further be referred to as user equipments, terminals, mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The mobile stations in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the mobile stations within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for mobile stations. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies. The evolution of UTRAN is commonly referred to as the Evolved-UTRAN (E-UTRAN) or LTE.
In the context of this disclosure, the expression DownLink (DL) is used for the transmission path from the base station to the mobile station. The expression UpLink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
Improved support for heterogeneous network operations is part of the ongoing enhancements of LTE specification of 3GPP LTE Rel-10. In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. One example of such deployments is where pico cells are deployed within the coverage area of a macro cell. A pico cell is a small cellular base station typically covering a small area. Thus, the small cellular base station transmits at a low power. Accordingly, the small cellular base station may be referred to as a low power node. Other examples of low power nodes in heterogeneous networks are home base stations and relays. As will be discussed in the following, the large difference in output power, e.g. 46 dBm in macro cells and less than 30 dBm in pico cells, results in a different interference situation than what is seen in networks where all base stations have the same output power.
The aim of deploying low power nodes such as pico base stations within the macro coverage area is to improve system capacity by means of cell splitting gains as well as to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployments are in particular effective to cover traffic hotspots, i.e. small geographical areas with high user densities served by e.g. pico cells, and they represent an alternative deployment to denser macro networks.
FIG. 1 depicts an example of macro and pico cell deployment in a heterogeneous network 100 comprising a macro cell 110 and three pico cells 120. The most basic means to operate a heterogeneous network is to apply frequency separation between the different layers, i.e. between the macro cell 110 and the pico cells 120 in the heterogeneous network 100 in FIG. 1. The frequency separation between the different layers is obtained by allowing the different layers to operate on different non-overlapping carrier frequencies. In this manner, any interference between the layers of cells is avoided. With no macro cell interference towards the pico cells 120 in FIG. 1, cell splitting gains are achieved when all resources can simultaneously be used by the pico cells. The drawback of operating layers on different carrier frequencies is that it may lead to resource-utilization inefficiency. For example, with low activities in the pico cells 120, it may be more efficient to use all carrier frequencies in the macro cell 110 and then basically switch off the pico cells 120. However, the split of carrier frequencies across layers is typically performed in a static manner.
Another related means to operate a heterogeneous network is to share radio resources on same carrier frequencies by coordinating transmissions across macro and pico cells. This type of coordination refers to as Inter-Cell Interference Coordination (ICIC) in which certain radio resources are allocated for the macro cells during some time period whereas the remaining resources can be accessed by the pico cells without interference from the macro cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the above mentioned split of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between nodes in the heterogeneous network. In LTE, an X2 interface has been specified in order to exchange different types of information between base station nodes. One example of such information exchange is that a base station can inform other base stations that it will reduce it's transmit power on certain resources.
Time synchronization between base station nodes is required to ensure that ICIC across layers will work efficiently in heterogeneous networks. This is in particular of importance for time domain based ICIC schemes where resources are shared in time on same carrier.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform spread OFDM (DFT-spread OFDM) in the uplink. In OFDM transmissions, set of modulated symbols is transmitted over narrowband and orthogonal subcarriers, where the number of subcarriers defines the transmission bandwidth of the ODFM signal. In DFT-spread OFDM, the set of modulated symbols is first pre-coded before generating the OFDM signal, where the pre-coding aim to provide power characteristics of the OFDM signal suitable for transmit power limited terminals. A basic LTE physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 2, where each resource element corresponds to one subcarrier during one OFDM symbol interval. Part of the OFDM symbol interval is a cyclic prefix introduced to mitigate inter-symbol interference. LTE supports two cyclic prefix lengths, commonly referred to as the normal and extended cyclic prefix, respectively.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes of 1 ms. A subframe is divided into two slots, each of 0.5 ms time duration. Each slot comprises of either 6 or 7 OFDM symbols depending on the selected cyclic prefix length.
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive resource blocks represent a resource block pair and corresponds to the time interval upon which scheduling operates.
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits assignments and/or grants to certain user equipments via the Physical Downlink Control Channel (PDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans over the whole system bandwidth. A user equipment that has decoded downlink control information, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the user equipment. In LTE, data is carried by the physical downlink shared channel (PDSCH).
Demodulation of sent data requires estimation of the radio channel which is done by using transmitted reference symbols, i.e. symbols known by the receiver. In LTE, cell specific reference symbols are transmitted in all downlink subframes and in addition to assist downlink channel estimation they are also used for mobility measurements performed by the user equipments. LTE supports also user equipment specific reference symbols aimed only for assisting channel estimation for demodulation purposes.
The length of the control region, which can vary on subframe basis, is conveyed in a Physical Control Format Indicator CHannel (PCFICH). The PCFICH is transmitted within control region, at locations known by user equipments. After a user equipment has decoded the PCFICH, it thus knows the size of the control region and in which OFDM symbol the data transmission starts.
Also transmitted in the control region is the Physical Hybrid-ARQ Indicator Channel. This channel carries Acknowledgement/Negative Acknowledgment (ACK/NACK) responses to a user equipment to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
Before an LTE user equipment can communicate with an LTE network it first has to find and acquire synchronization to a cell within the network, i.e. performing cell search. Then it has to receive and decode system information needed to communicate with and operate properly within the cell, and finally access the cell by means of the so-called random-access procedure.
FIG. 3 depicts uplink and downlink coverage in a mixed cell scenario. In order to support mobility, a user equipment needs to continuously search for, synchronize to, and estimate the reception quality of both its serving/camping cell and neighbor cells. The reception quality of the neighbor cells, in relation to the reception quality of the current cell, is then evaluated in order to conclude if a handover for user equipments in connected mode, or cell re-selection for user equipments in idle mode, should be carried out. The procedure for changing cell depends on which of the two Radio Resource Control (RRC) states a user equipment is in: connected mode or idle mode. In idle mode, mobility is controlled by the user equipment, referring to cell re-selection, whereas in connected mode mobility is controlled by the network, referring to handover. For user equipments in connected mode, the handover decision is taken by the network based on measurement reports provided by the user equipment. Examples of such reports are Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). Depending on how these measurements, possibly complemented by a configurable offset, are used the user equipment can be connected to the cell with the strongest received power, or the cell with the best path gain, or a combination of the two. These do not result in the same selected cell as the base station output powers of cells of different type are different. This is sometimes referred to as link imbalance. For example, the output power of a pico base station or a relay is in the order of 30 dBm or less, while a macro base station can have an output power of 46 dBm. Consequently, even in the proximity of the pico cell, the downlink signal strength from the macro cell can be larger than that of the pico cell. From a downlink perspective, it is better to select cell based on based on downlink received power, whereas from an uplink perspective, it would be better to select cell based on the path loss. The cell selection approaches are illustrated in FIG. 3.
Hence, in the above scenario, it might be a better case, from a system perspective to connect to the pico cell even if the macro downlink is much stronger than the pico cell downlink. However, ICIC across layers would be needed when a user equipment 300 operate within the region of the UL border and the DL border, i.e. the link imbalance zone 310 depicted in FIG. 3.
A user equipment in connected mode may be requested by the base station to perform Channel State Information (CSI) reporting, e.g., reporting a suitable Rank Indicator (RI), one or more Precoding Matrix Indices (PMIs) and a Channel Quality Indicator (CQI). A CQI report reflects the instantaneous radio quality in a certain downlink subframe observed by the user equipment whereas RI and PMI reports provide the network with user equipment suggestions of parameter settings for Multiple-Input Multiple-Output (MIMO) transmissions. Other types of CSI are also conceivable including explicit channel feedback and interference covariance feedback.